The broad aim of the proposed research project is to elucidate the mechanisms of transcriptional activation at the atomic level using a combination of molecular biology and X-ray crystallography approaches. Initially, I will focus on determining the X-ray structure of human TAF(II)55 as well as the human TAF(II)55-TAF(II)250 complex, a part of the pre-initiation complex thought to be a target for a number of transcription regulatory proteins. I will then carry out a series of experiments to quantitatively characterize the previously reported interactions of known regulators of transcription, such as Spl and YY1, and map the domains of these proteins that interact directly with the TAF(II)55-TAF(II)250 heterodimer. In order to characterize these interactions in greater detail, I will attempt to crystallize the complexes composed of both TAFs as well as the interacting domains of the transcriptional activators, and solve their structures. The studies will help us understand the mechanisms by which selected, well-characterized transcriptional activators involved in cancer and arthritis interact with the basal factors in the pre-initiation complex to activate transcription.